1. Field of the Invention
The present invention relates to an optical amplifier and relates a method and an apparatus for monitoring an optical fiber transmission path, and more particularly to fault localization of an optical fiber transmission path using an optical time domain reflectometry (hereinafter, referred to as OTDR).
2. Description of the Related Prior Art
As one of methods for localizing a fault in an optical fiber transmission path, an OTDR is publicly known (THE JOURNAL OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS OF JAPAN, Vol.J63-B No.2, February 1998). The OTDR is a method for observing the state of an optical fiber, comprising a step of inputting a short pulse light into the optical fiber and a step of measuring a time distribution of the level of a backscattered light returned by Rayleigh scattering occurring in the optical fiber.
A conventional optical amplification relay transmission system to which an OTDR is applied is provided with an optical transmission path comprising a pair of optical fibers which are respectively an up optical fiber and a down optical fiber. This optical fiber transmission path has an optical amplifier apparatus provided with an optical amplifier for amplifying an up signal light and an optical amplifier for amplifying a down signal light. This optical fiber transmission path is provided between end stations performing transmission and reception of signal lights. The end station is provided with a transmitter and a receiver for a probe light of an optical fiber. An optical amplifier as described above is ordinarily provided with an optical isolator in order to prevent a returning light generated by reflection at the output side. Accordingly, this optical isolator prevents every returning light including a backscattered light. The optical amplifier is therefore provided with an optical bypass circuit which is composed of two optical couplers and connects an up and a down optical fiber transmission path (Y. Horiuchi et. al. ECOC ""93, MoCl.3, September 1993, and Proceedings of the Autumn Convention of the Institute of Electronics, Information and Communication Engineers of Japan, B-619, 1991). A returning light coming back through an up optical fiber transmission path is separated by an optical coupler before an optical amplifier and is inserted into a down optical fiber transmission path by another optical coupler. This returning light is returned to an end station through the down optical fiber transmission path, is separated by an optical coupler inside the end station, and received. At this time, only the wavelength component of a probe light is selected and received. The backscattered light of the probe light out of the received light is measured. A time change in level of the backscattered light is represented as shown by a dashed line in a graph of FIG. 7. On the basis of this result, a fault of an optical fiber between the optical amplifier apparatuses in the up optical fiber transmission path is measured.
As described above, in an optical relay transmission system using a conventional optical amplifier, a returning light from an up optical fiber transmission path passes through two optical couplers and is inputted into an opposite down optical fiber transmission path and comes back to an end station. Since loss of the returning light in the optical couplers is very large, the level of a backscattered light to be received is very small and it is difficult to observe the optical fiber in high accuracy. And a conventional fault localization method needs an opposite optical fiber transmission path.
Accordingly, an object of the present invention is to provide a simple apparatus and method capable of performing a high-accuracy fault localization of an optical fiber transmission path and an optical amplifier used in this apparatus.
An optical amplifier of the present invention comprises a light amplifying portion for amplifying an input light, an optical circulator provided at the output side of it, and a wavelength selectively reflecting device connected to this optical circulator. In this optical amplifier, the optical circulator sends returning lights to the wavelength selectively reflecting device, which reflects a returning light having a specific wavelength. This reflected light is returned to the light amplifying portion. A method for localizing a fault in an optical fiber transmission path comprises the following steps; that is to say, a step of sending out a probe light having a specific wavelength into an optical fiber transmission path, a step of selecting a returning light of the said probe light by means of an optical circulator arranged in the optical fiber transmission path and a wavelength selectively reflecting device connected to this optical circulator, and a step of receiving the returning light of the probe light through the optical fiber transmission path. After this, a time change in level of a backscattered light in the returning light is observed. An apparatus for monitoring an optical fiber transmission path comprises a light source for outputting a probe light having a specific wavelength, an optical receiver for receiving a returning light of the probe light, an optical coupler for coupling the optical fiber transmission path with the light source and the optical receiver, and an optical amplifier arranged in the optical fiber transmission path. The optical amplifier is provided with a light amplifying portion, an optical circulator provided at the output side of the light amplifying portion, and a wavelength selectively reflecting device connected to this optical circulator. The wavelength selectively reflecting device reflects selectively a returning light of a probe light, and the optical circulator outputs this reflected light to an optical fiber transmission path. An optical fiber transmission system comprises end stations transmitting and receiving signal lights, an optical fiber transmission path provided between the said end stations, and the above-mentioned optical amplifier apparatuses provided in the said optical fiber transmission path. The end station comprises a light source outputting the above-mentioned probe light having a specific wavelength, an optical receiver for receiving a returning light of this probe light, and an optical coupler for coupling the optical fiber transmission path with the light source and the optical receiver. A light amplifying portion in said optical amplifier is provided with a rare-earth-doped optical fiber, a pumping light source, and an optical coupler for inputting an excitation light into the rare-earth-doped optical fiber. The wavelength selectively reflecting device is selected from an optical fiber grating, a grating substrate, and a waveguide grating. In the above-mentioned invention, it is possible to localize a fault in an optical fiber transmission path in high accuracy by means of a simple apparatus.